1. Field of the Invention
The invention relates generally to electronic circuits and, more particularly, to circuits for performing a reset scheme in devices having multiple power supplies.
2. Description of the Related Art
Low voltage reset schemes are used in devices required to perform operations from the time a low voltage indication is generated to the time power is lost for all operational purposes. These operations may include protecting values in some important status registers by storing them to a non-volatile storage on board or device, protecting a data being written to a non-volatile memory, and other related functions. These low voltage reset schemes are generally used in microcontroller-based circuits. In one conventional embodiment, certain 5V microcontroller devices have used these kinds of circuit to protect on-chip flash memory when power falls below 4.1V.
FIG. 1 is a block diagram of a conventional low voltage inhibits reset signal generation circuit 1. As shown, the conventional low voltage reset scheme is based on comparing a resistor divided value of the power supply (Vdd) with a stable reference like a bandgap voltage (Vbg). The low voltage reset signal LVI_reset is coupled to a reset logic for issuing a “reset” signal to a pin depending on the voltage level of the supply voltage (Vdd).
The conventional solution in FIG. 1 comprises a voltage reference generated by a resistor divider network 2, where at the top of the network is a power supply (Vdd) and the output of the resistor network comprises a voltage reference signal 3. The voltage reference signal 3 is coupled to the negative input of a comparator 5. The positive input of the comparator 5 is coupled to a bandgap reference voltage Vbg, and the output of the comparator 5 forms a low voltage indicator reset signal LVI_reset.
A disadvantage of the conventional solution is that the low voltage inhibit reset signal generation circuit 1 cannot support devices having multiple power supply levels (multiple Vdds). When there are multiple Vdds on a chip, a reset circuit needs to be activated when one or more of the Vdds is lost. It is also desirable to have a user select whether to activate reset on the device when one specific Vdd is lost or when any one of the Vdds is lost.
It would be desirable to have a device capable of supporting multiple power supply levels because the device may need to interface with other devices operating under different power supply levels on a board.